Study reveals significant limitations to ultrasound diagnosis among obese pregnant women

HOUSTON -- (November 18, 2009) -- Ultrasound performed in the second trimester of pregnancy on obese mothers is less likely to detect structural anomalies or other markers of chromosomal disease in the developing fetus, said a consortium of researchers led by one from Baylor College of Medicine in a report that appears in the current issue of the journal Prenatal Diagnosis.

"The incidence of obesity in pregnancy has steadily increased over the past two decades. Interestingly, multiple population studies have demonstrated that obese women are more likely to have babies affected by neural tube (failure of the brain, spinal cord and/or their coverings to develop normally) and heart abnormalities. Unfortunately, we are hindered by the fact that when we do ultrasounds on obese mothers, we are more limited in our ability to detect these abnormalities in the developing fetus," said Dr. Kjersti Aagaard, assistant professor of obstetrics and gynecology at BCM and the study's first author.

Harder to find abnormalities

This could result in missing some abnormalities that would be found in the offspring of less obese pregnant women, she said.

"Our practical experience is that the depth of the body fat layer limits accurate sonographic visualization of both fetal and maternal structures in pregnancy. We sought in this study to objectively describe to what extent our common observations are actually true."

In the study, she and colleagues evaluated data from 8,555 women for whom they had complete body mass index (BMI) information and were among the 38,033 women enrolled in the First and Second Trimester Evaluation of Risk (FaSTER) trial that took place at multiple sites in the United States. All the women who underwent a detailed ultrasound in the second trimester of their pregnancies did so with recorded data detailing sonographic detection of both major and multiple minor fetal abnormalities and so called 'soft markers' of chromosomal abnormalities.

There were 62 babies with Down syndrome born to mothers in the group, and an additional 490 babies with stomach and intestinal abnormalities, heart defects, and other significant abnormalities for an overall 11.5 percent "positive" sonographic detection rate. Although all these abnormalities were detected before birth because doctors did many different kinds of tests, markers that could have detected the Down syndrome and other signs of abnormalities by ultrasound were missed more often in obese mothers.

Heart defects

For example, although fetal heart defects are more common, ultrasound was also less likely to pick up heart abnormalities in the fetuses of obese women, said the authors. The sound waves have to travel through more tissue in the ultrasound, often making it difficult to obtain a clear image.

"When we calculated a 'missed diagnosis rate' for all our sonographic findings, it was uniformly elevated in association with maternal obesity," Aagaard said.

Inform mothers

The researchers said obese mothers should be told about the limitations of the technology.

"Taken together, our data reiterate the notion that with increasing prevalence of obesity among reproductive-aged women, the prenatal sonographic examination may have inherent limitations which cannot be ascribed to the sonographer, physician, equipment or institution," the researchers wrote.

The implications of Aagaard's research is that for the prenatal diagnosis of genetic conditions, such as Down syndrome, obese pregnant women may wish to consider the greater reliability of invasive diagnostic tests such as amniocentesis and chorionic villus sampling (CVS).

Others who took part in this work include T. Flint Porter of the University of Utah in Salt Lake; Fergal D. Malone of the Royal College of Surgeons in Dublin, Ireland; David A. Nyberg of the Swedish Medical Center in Seattle; Jamie Collins of DM-STAT in Malden, Mass.; Christine H. Comstock of William Beaumont Hospital in Royal Oak, Mich.; Gary Hankins of The University of Texas Medical Branch in Galveston; Keith Eddleman of Mount Sinai Medical Center in New York; Lorraine Dugoff of the University of Colorado Health Sciences Center in Denver; Honor M. Wolfe of the University of North Carolina at Chapel Hill and Mary E. D'alton of Columbia University in New York. Aagaard was at the University of Utah as a FaSTER trial investigator for much of the study, and completed the analysis after moving to BCM in 2007.

Funding for this work came from the National Institutes of Health and the National Institutes of Child Health and Human Development.

For more information on research at Baylor College of Medicine, please go to www.bcm.edu/fromthelab and www.bcm.edu/findings.